Piston pump for high-pressure fuel generation

ABSTRACT

A high-pressure piston pump with a plurality of pump elements is connected on the intake side to a pump that pumps fuel at low pressure in a regulated quantity. Each pump element is preceded on the intake side by a first and a second check valve in series. The first check valve opens counter to spring force in the intake phase of the pump element and closes in the pumping phase. The second check valve opens counter to spring force and closes, reinforced by spring force, at pressures of the fuel supplied by the pump that are higher than the negative pressure generated in a cylinder chamber of the pump element. The second check valve takes on the metering of the fuel supplied to the pump element; the first check valve blocks off the cylinder chamber counter to the check valve in the pumping phase, whereby the piston pump serves to supply fuel at high pressure in fuel injection systems of internal combustion engines, especially in a common rail injection system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 99/03643 filed onNov. 16, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention directed to a piston pump and more particularly to apiston pump for supplying high pressure fuel to a fuel injection is aninternal combustion engine.

2. Description of the Prior Art

One such piston pump is known (German patent disclosure De 42 13 798 A1)that has three pump elements which are each connected on the intake sideto a low-pressure supply, via a spring-loaded check valve. The knownpiston pump is by its design a constant pump, whose pump elements areset to a maximum required volumetric flow in a fuel injection system. Inquantity regulation of the fuel flow on the low-pressure side, however,the disadvantage arises that at a volumetric flow smaller than themaximum volumetric flow, unequal filling of the various pump elementsensues because of variations in the check valves on the intake side. Thereason for this is that the intake-side check valve of the pump elementsset for a small opening stroke is opened during the intake stroke of thepump piston and during part of the pumping stroke. Overlaps in theopening times of the intake valves of other pump elements can occur.However, since at a small volumetric flow the pressure in thelow-pressure system is quite low and decreases further upon filling of apump element, if the opening time of one intake valve is too long, theresult can be incomplete or entirely absent filling of another pumpelement. In the high-pressure part of the fuel injection system,however, this causes pressure fluctuations, which adversely affect theoperation of the internal combustion engine connected to it.

From British Patent GB 564 725, a piston pump with two intake-side checkvalves connected in series is known. The check valves, which arestructurally identical, have a ball that is not spring-loaded as theirclosing member, which assumes its closing position by gravity. With thedual disposition of the intake-side check valve, the intent is toachieve improved tightness and effectiveness of the pump.

SUMMARY OF THE INVENTION

The piston pump of the invention has the advantage over the prior artthat the partial filling of a given pump element no longer dependsessentially on the cooperation among the feed pressure of the feed pump,the spring force of the first check valve, and the negative pressuregenerated by the pump piston; instead, the duration of filling of thepump element is determined by the second check valve, which isessentially loaded only by feed pressure and spring force, and thesecond check valve also limits the filling when the volumetric flow ofthe supplied fuel is low, while the first check valve in the compressionphase of the pumping process now serves essentially only to block offthe cylinder chamber of the pump element from the second check valve.The fuel metering operation is thus no longer determined by the durationof opening of the first check valve. The onset and end of the meteringoperation are initiated and defined according to the invention by thefeed pressure of the fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

One exemplary embodiment of the invention is described in further detailherein below, taken in conjunction with the drawings, in which:

FIG. 1 shows a hydraulic circuit diagram of part of a fuel injectionsystem having a piston pump for supplying fuel at high pressure to thesystem, and

FIG. 2 is a section through a pump element of the piston pump.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The hydraulic circuit diagram in FIG. 1 shows a pump assembly forgenerating high fuel pressure for a fuel injection system used ininternal combustion engines, especially a common rail injection system.The pump assembly has a low-pressure feed pump 1, which on the intakeside is connected to a fuel tank 2 that for instance contains Dieselfuel. On the compression side, a supply line 3 in which a flowregulating valve 4 is disposed extends from the feed pump

The pump assembly furthermore has a high-pressure piston pump 6, withthree pump elements 7; each pump element includes one pump piston 8 in acylinder chamber 9, one first and one check valve 10 and 11 on theintake side, and one pressure valve 12 on the compression side. The pumppistons 8, disposed at an angular spacing of 120°, are braced by springforce on a stroke ring 13, which can be driven by an eccentric shaft 14.

The supply line 3 branches downstream of the flow regulating valve 4 andis connected to the second check valve 11 of each of the pump elements7. The two check valves 10 and 11, opening in the direction of thecylinder chamber 9 counter to spring force, are connected in series,with the first check valve 10 located near the cylinder chamber in theapplicable branch of the supply line. From the pressure valves 12 of thepump elements 7, line branches of a high-pressure fuel line 15 extend toa high-pressure fuel reservoir, or common rail, of the fuel injectionsystem.

The low-pressure feed pump 1 and the high-pressure piston pump 6 areconstant pumps. Consumption-dependent quantity regulation of the fuelflow from the feed pump 1 to the piston pump 6 is effected by means ofthe flow regulating valve 4. In the simplified hydraulic circuit diagramof FIG. 1, none of the pressure regulating and limiting valves, returnlines, and fuel filters that all belong to the pump assembly are shown.

In the longitudinal section through a pump element 7 shown in FIG. 2, apump piston 8 can be seen in a cylinder chamber 9 of a housing 17. Thepressure valve 12 of the pump element 7 is connected to the cylinderchamber 9. the cylinder chamber 9 is closed off by a valve plate 18 inthe form of an annular disk. The valve plate is held down by a housingcomponent 19, in the form of a closure screw that is screwed into thehousing 17. The housing component 19, with a sealing edge 20, engagesthe side of the valve plate 18 remote from the cylinder chamber, and itis sealed off on its circumference from the housing 17 by a sealing ring21. The valve plate 18 is surrounded on its circumference by an annularchamber 22, into which a line branch of the supply line 3 discharges.

The valve plate is provided with a graduated through bore 24 thatextends coaxially to the cylinder chamber 9. A blind bore 25 is embodiedin the housing component 19, coaxially with the through bore 24. Thevalve plate is furthermore provided with a radially extending branchconduit 26, which extends between the circumferential annular chamber 22of the housing 10 and the graduated through bore 24. The branch conduit26 discharges into a bore portion 27 of the through bore 24 that islocated between an annular collar 28 toward the cylinder chamber and abore portion 29 of the valve plate 18 remote from the cylinder chamber.

Toward the cylinder chamber, a hollow-conical valve seat 31 of the firstcheck valve 10 is embodied on the annular collar 28 of the valve plate18. This check valve has a platelike closing member 32, which is definedconically toward the valve seat 31. Because the cone angles differ fromone another, the closing member 32 and the valve seat 31 touch along anedge whose diameter is at the same time the inside diameter of theannular collar 28. In a departure from the exemplary embodiment, it isalso possible for the closing member 32 to be merely disk-shaped and tocooperated with the valve plate 18 by way of a flat valve seat 31. Ashaft 33 extending from the closing member 32 penetrates the throughbore 24 of the valve plate 18 with spacing and ends in the blind bore 25of the housing component 20.

A hollow-conical valve seat 35 of the second check valve 11 is embodiedon the side of the annular collar 28 remote from the cylinder chamber. Aclosing member 36 in the form of a sleeve is assigned to the secondcheck valve, and its bottom 37 has a conical contour that cooperateswith the valve seat 35. By suitably selected cone angles, the sealingdiameter of the second check valve 11 matches the inside diameter of theannular collar 28. The sleevelike closing member 36 of the second checkvalve 11 is guided largely in pressure-tight fashion in the bore portion29 of the valve plate 18 remote from the cylinder chamber, and itextends into the blind bore 25 of the housing component 19. Inside theblind bore 25, there is a prestressed compression spring 38, which isbraced at one end, toward the bottom, on the closing member 36 and onthe other on the bottom of the bore portion 25 on the housing component19. The sleevelike closing member 36, with its bottom 37, surrounds theshaft 33 of the closing member 32 with radial play. A prestressedcompression spring 39 is received on the shaft 33, on one end engagingthe side of the sleeve bottom 27 remote from the cylinder chamber and onthe other engaging a stop 40 on the closing member shaft. The twocompression springs 38 and 39 each exert a closing force on the checkvalve 10 and 11, respectively, associated with them.

To explain the mode of operation of the two intake-side check valves 10and 11, let it be assumed that the first check valve 10 is set for anopening pressure of 0.3 bar, and the second check valve 11 is set for anopening pressure of 1 bar. Let it also be assumed that both check valves10 and 11 are in their closing position. The pressure of the fuel feedflow, pumped by the feed pump 1 and metered in quantity-regulatedfashion by the flow regulating valve 4, prevails in the bore portion 27of the through bore 24 in the valve plate 18 upstream of the closedsecond check valve 11. During the intake stroke of the pump piston 8, anegative pressure occurs in the cylinder chamber 9 and overcomes thespring force of the compression spring 39 and shifts the first checkvalve 10 into the open position (as shown). While the blind bore in thehousing component 20 is pressure-relieved toward the cylinder chamber 9,the pressure of the fuel prevailing in the bore portion 27 of thethrough bore 24 of the valve plate 18 is exerted on a circular-annulareffective area of the closing member 36 of the second check valve 11;this effective area is defined on one side by the sealing diameter ofthe valve seat 35 and on the other by the sealing diameter of the boreportion 29. If the pressure of the fuel exceeds the prestressing forceof the compression spring 38 that is exerted on the closing member 36,then the second check valve 11 opens, and fuel flows into the cylinderchamber 9 of the pump element 7. The fuel pressure, which is dependentin its magnitude on the feed flow supplied, collapses upstream of thesecond check valve 11 during the filling operation, causing this checkvalve to shift from the open position, shown, to the closing position.The metering of the fuel quantity in the cylinder chamber 9 of the pumpelement 7 is thus effected by the second check valve 11. During theensuing pumping phase of the pump piston 8, the pressure in the cylinderchamber 9 rises, and the first check valve 10 assumes its closingposition. The second check valve 11, which functions in the oppositedirection from the first check valve 10, is thus protected against beingforced open by the fuel compressed by the pump piston 8. During theclosing position of the second check valve 11, the pressure of the fuelpumped by the feed pump 1 increases again, and brings about thedescribed valve function at the next pump element 7 to enter the intakephase. At the end of the pumping phase of the pump element 7, thepressure valve 12 opens, and the compressed fuel is expelled into thehigh-pressure line 15.

What is claimed is:
 1. A piston pump (6) including a plurality of pumpelements (7) for supplying fuel at high pressure in fuel injectionsystems of internal combustion engines, in particular in a common railinjection system, wherein the pump elements (7) are connected on theintake side to a pump (1) that pumps fuel at low pressure in a regulatedquantity, each pump element (7) is preceded on the intake side by afirst check valve (10), with which the supply of fuel into a cylinderchamber (9) of the pump element (7), which chamber has a pump piston(8), is controllable, the first check valve (10) opens counter to springforce in the intake phase of the pump element (7) and closes in thepumping phase, and wherein a second check valve (11) precedes the firstcheck valve (11) on the inflow side; and the second check valve (11)opens counter to spring force and closes, reinforced by spring force, atpressures of the fuel supplied by the pump (1) that are higher than thenegative pressure generated in a cylinder chamber (9) of the pumpelement (7).
 2. The piston pump of claim 1, wherein a valve plate (18)adjoining the cylinder chamber (9) is provided; a graduated through bore(24) is embodied in the valve plate (18); on an annular collar (28) ofthe through bore (24), a first valve seat (31) for the engagement of aclosing member (32) of the first check valve (10) is embodied on theside toward the cylinder chamber, and on the side remote from thecylinder chamber, a second valve seat is embodied for the engagement ofa closing member (36) of the second check valve (11); in the valve plate(18), on the side remote from the cylinder chamber, the closing member(36) of the second check valve (11) is guided longitudinallydisplaceably, in largely pressure-tight fashion, in the through bore(24), and the diameter of the guiding bore portion (29) is greater thanthe sealing diameter of the associated valve seat (35); fuel is pumpedby the feed pump (1) into a bore portion (27) of the through bore (24)that is located between the valve seat (35) of the second check valve(11) and the guiding bore portion (29).
 3. The piston pump of claim 1,wherein closing member (36) of the second check valve (11) takes theform of a sleeve, whose bottom (37) that engages the associated valveseat (35) is penetrated coaxially with play by a shaft (33) of theclosing member (32) of the first check valve (10).
 4. The piston pump ofclaim 3, wherein the closing member (35) of the second check valve (11)is braced, remote from the cylinder chamber, by a prestressedcompression spring (38) against a housing component (19) of the pumpelement (7).
 5. The piston pump of claim 3, wherein a prestressedcompression spring (39) is received on the shaft (33) of the closingmember (32) of the first check valve (10), on one side engaging the sideof the sleeve bottom (37) of the second check valve (11) remote from thecylinder chamber and on the other engaging a stop (40) on the closingmember shaft (33).